Coating process



June 2, 1970 s. BARANOW ET AL COATING PROCESS 2 Sheets-Sheet 1 Filed May 5, 1967 INVENTORS Sanford Borcmow William R. Freeman,Jr. BYM vl- ATTORNEY June 2, 1970 BARANOW ET AL 3,515,095

COATING PROCES 5 Filed May 5, 1967 2 Sheets-Sheet 2 INVENTOIB Sanfbrd Boranow William R. Freeman,dr.

I6 BY J. 74%

ATTOR NE Y United States Patent 3,515,095 COATING PROCESS Sanford Baranow, Woodbridge, and William R. Freeman, Jr., Easton, Conn., assignors to Avco Corporation, Stratford, Conn., a corporation of Delaware Filed May 3, 1967, Ser. No. 635,893

Int. Cl. C23c 1/08 US. Cl. 118-48 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to means for pack aluminizing articles composed of nickelor cobalt-base super-alloys in order to provide the articles with an outer coating of NiAl or CoAl. By suitably adapting the means, it is also possible to obtain either coatings of uniform thickness or coatings of non-uniform thickness in which the thickness may taper in a desired pattern as hereinafter described.

The invention will be more fully understood from the description which follows taken in conjunction with the drawings in which FIG. 1 is a schematic plan view showing one manner of loading of a furnace unit to produce a desired tapered thickness in the coating of a turbine blade, taken on a plane corresponding to BB in FIG. 2, 3 or 4;

FIGS. 2, 3 and 4 are schematic views showing other arrangements for obtaining non-uniform coatings on the processed articles, taken on a plane corresponding to AA of FIG. 1;

FIG. 5 is a view like FIG. 1 showing a loading pattern wherein a more uniform coating is obtained;

FIG. 6 is a schematic view in section of a turbine blade coated by the process of this invention;

FIG. 7 is a fragmentary view, greatly magnified, schematically depicting a coated produce produced by the process of this invention; and

FIGS. 8 and 9 are schematic plan and elevation views of a vacuum furnace arrangement, partly broken away.

The furnace shown schematically in FIGS. 8 and 9 comprises an enclosure 10 provided with heating means (not shown), which means are readily controllable in known manner to produce any desired temperature in the furnace and to maintain the furnace and its contents at that temperature. Extending through a wall 12 of enclosure 10 is a conduit 14 which connects the enclosure with a source of vacuum shown here as a diffusion pump 16 and a mechanical pump 18 connected in series through conduit 20. Another conduit 22 connects the inside of enclosure 10 to a source of argon or other inert gas, stored in a reservoir 24. The enclosure of heating chamber 10 is provided with a hearth 30 on which there may be positioned one or more containers 32 such as those shown in FIGS. 1 to 5 as cans of nickel or steel open at the top and adapted to be covered by a loosely fitting cover 34 having one or more perforations 36 therein to permit free passage of gases between the heating enclosure 10 and the individual containers 32 and to permit the contents of the individual containers to be subjected to pressures lower than atmospheric, when the furnace 10 is evacuated, or to atmospheres of desired composition introduced into the furnace through conduit 22.

As shown in FIGS. 2, 3 and 4, the containers 32 and 32' and the disposition of the articles being aluminized ice may be varied to produce non-uniformity in the thickness of the aluminum coating deposited thereon. For example, a thickness variation may be achieved by distributing the articles radially around a metal rod or arbor 40 as shown in FIG. 1. Arbor 40 may be welded to the bottom of container 32. When the enclosure 10 is heated, heat flows toward the article through the walls of the containers 32, 32' as shown by arrows 42 and additional heat flows toward the articles from the centrally disposed metal arbor 40, 40'. The temperature of any point on the surface of the article depends on the temperature within the heating enclosure 10, the apparent thermal diifusivity of pack material (chiefly a function of furnace pressure), and also depends on the distance from the point on the surface of article to the surface of container 32 and to the surface of arbor 40. The arrangement in FIG. 1 has been found to be very desirable for coating turbine blades, even those as small as one-half inch in width, the leading edges of which require a heavier coating than the trailing edges. The leading edges are placed nearest to the wall of the retort 32 and the trailing edges are disposed adjacent to the metal arbor 40. As a result of the difference in temperature along the surface of the articles, accentuated by the use of a vacuum which results in a low heat transfer rate through the coating pack, more coating material deposits on the warmer leading edges than deposits on the cooler trailing edge, because a thicker coating deposits on the warmest surface of the article and the coating which deposits on the coolest surface of the article is thinner, the total amount of the deposit at any point depending on both the temperature and the time interval that point on the article surface is at coating depositing conditions.

FIG. 6 shows a turbine blade on which the coating varied from a minimum of 1.75 mils at one end to a maximum 3.5 mils at the other end, due to the variation in the time-temperature relationship, the blade measuring 0.75 inch from the leading edge to the trailing edge.

In FIG. 2, rod 40 is shown tapered from a thicker top to a thinner bottom, in order to produce a heating pattern varying axially along the container, the taper being exaggerated for purpose of illustration.

Other means and other loading patterns for controlling the heat transferred to articles being coated within the furnace will be readily apparent to persons skilled in the art. Forexample, in FIG. 3, the articles are packed at distances varying from the inner wall of can 32 and in FIG. 4, the container 32 is tapered rather than cylindrical.

The diffusion coating process is performed in a moderate vacuum in unsealed containers utilizing as the source of the aluminum in the deposited coating, a commercially available alloy preferably consisting of 56% Cr and 44% Al, by weight. Alloys having less than about 40% by weight of aluminum do not produce a good NiAl or CoAl deposit and those having too high an aluminum content melt at temperatures below the temperature limit for this coating process. Consequently, a minimum of about 40% aluminum and a maximum of about 48% aluminum represents a somewhat critical range for the starting material. The Cr-Al alloy is crushed to a mesh size between 50 and +325 (Tyler standard sieve) and then is mixed with about one half its weight of A1 0 powder of about the same size. The mixture is poured into cans 32 and the articles 50 to be coated are placed on the bed of the mixture and then covered with additional mixture. The reason for the use of 50 and +325 mesh powder will be explained later.

It appears that the use of an alloy with a high aluminum content and a melting point well above the temperature at which it decomposes when the alloy is heated in a vacuum along with the use of a lower pressure during the soaking step described below results in vaporization of some of the aluminum from the alloy powder anddeposition of the vapor onto the articles embedded in the pack material. Competition always exists between back streaming furnace contaminants (chiefly water vapor) and the aluminum vapor so that thinner, aluminum poor coatings are derived when coarser and, therefore, less surface active powders are utilized. Finer than 325 mesh powders may be used, but constitute a health hazard. It will be noted that no activator or halide salt, such as ammonium chloride commonly used in this art, is utilized in the present process. It has been found, however, that when a very small amount of ammonium chloride is included in the coating batch composition it functions to flush out any residual air from the coating pack during the initial heating of the pack during its vaporization and that it also increases the rate of heat transfer during the earlystages in which the pack is heated up from room temperature to about 1600 F.

By interrupting the heating after the pack has been heated up to about 1600 F., it has been established that all traces of the ammonium chloride have disappeared therefrom before any coating has taken place and that no halide remains to act as a carrier during the next step in the coating process.

An example of a heating cycle involves heating first to 1600 F. at a rate of roughly 1200 F. per hour while subjecting the heating furnace to a sub-atmospheric pressure by use of pumps 16 and 18; holding the pack at 1600 F. for 2 /2 hours at a pressure of microns or less, whereby absorbed air or other contaminating gas are eliminated from the pack; and then heating to 1900 F. in about minutes and holding at this temperature for 5 to 10 hours. Thereafter, the pack is cooled to 1200 F. while the vacuum is maintained. When the pack reaches 1200 F., the vacuum line 14 is closed and argon or other inert gas is admitted to the enclosure through line 22, whereby the pack and its contents are rapidly cooled to a temperature at which the coating is no longer affected by the atmosphere.

We claim:

1. An apparatus for producing a diffusion coating of 4 non-uniform thickness on the surface of metallic articles, said apparatus comprising:

(a) vessel adapted to contain an article to be diffusion coated;

(b) particulate coating material in said vessel adapted to have said article embedded therein while it is being heated, and adapted to position said article in a specific orientation relative to said apparatus;

(0) a first heating means to heat said vessel, the particulate material therein and the article embedded in said particulate material; and

(d) an additional heating means located within said vessel and including at least one heating element comprising a metal rod of gradually increasing diameter along its length which is embedded in said particulate coating material and positioned therein so as to direct heat toward a specific localized area of said article; V

said first heating means and said additional heating means providing heat to said article so as to maintain at least a first portion of the surface of said article at a temperature in excess of the average surface temperature of said article and a second portion of the surface of said article at a temperature less than the average surface temperature of said article.

2. The apparatus of claim 1 including in addition, means for maintaining a subatmospheric pressure in said vessel.

3. The apparatus of claim 1 including in addition, means for providing an inert atmosphere to said vessel.

References Cited UNITED STATES PATENTS 1,853,369 4/1932 Marshall. 2,009,820 7/1935 Shanklin 118-64 X 2,875,070 2/1959 Galmiche. 3,317,343 5/1967 Ieifreys.

MORRIS KAPLAN, Primary Examiner 

